How much water is right?
First, let’s discuss what water’s role is in the process of cement hydration. The cement (and cementitious materials like fly ash) in the concrete needs water to hydrate and form calcium-silicate-hydrate (C-S-H) which is the crystalline glue that holds the concrete together. The water is chemically bound (consumed) during the reaction with the cement at approximately 25 pounds of water to every 100 pounds of cement. Therefore, it could be said that a water-to-cement ratio, w/c (or water-to-cementitious materials ratio, w/cm) of 0.25 is needed.
But that’s not all of the water that is needed. Additional water becomes physically bound between the cement hydrates. So to have enough water to enable complete hydration of the cement, about 20 more pounds of water is needed for every 100 pounds of cement. Combined, this means you’ll need 45 pounds, resulting in a w/cm
of 0.45. Other studies have shown that an approximate ratio of 0.40 was necessary for complete hydration of the cement. But concrete rarely gets the benefit of complete cement hydration because of the lack of physical access of the water to the inner unhydrated cement particles.
But the reality is that lower w/cm values enhance the strength and durability of concrete even though not all of the cement may have been hydrated. The reason is that with more water in the mixture comes greater dispersion of the ingredients which means less bridging of the C-S-H crystals can take place. The resulting concrete is therefore less dense, lower in strength, and higher in permeability (resulting in lower durability).
The dilemma that exists between wanting lower w/cm values, which result in a denser concrete, and having enough water in the concrete mixture for adequate workability and to optimize hydration is explored further in an insightful article entitled “Curing and Hydration - Two half-truths don’t make a whole,” written by Ken Hover in the L & M Concrete News in 2002. In that article, Hover states that the solution to the problem is:
1. Restrict mixture water content to bring the cement grains close together, and
2. Apply effective curing measures to minimize water loss, and whenever possible water-curet o externally provide the water needed to sustain hydration.
In order to determine the correct amount of water as part of the design process, the Portland Cement Association’s “Design & Control of Concrete Mixtures” states that a properly proportioned concrete mix should have acceptable workability when fresh; durability, strength, and uniform appearance when hardened, and be economical to make.
Compressive strength and durability
When water is added to a load of concrete in excess of the design w/cm, the following performance characteristics of the concrete will be negatively affected:
- Compressive strength is lowered
- Resistance to freeze-thaw cycles is reduced
- Resistance to damage from sulfates in soil and water is lessened
- Permeability increases which reduces durability
- The ability to resist corrosion of reinforcing steel is reduced
Figure 2 and Table 1 (from Table 6.3.4, ACI 211.1-91) indicate a typical relationship between w/cm
and compressive strength but just as important may be the effect on the other performance characteristics of concrete related to durability. ACI 318, “Building Code Requirements for Structural Concrete,” uses the w/cm
ratio as the primary concrete mixture parameter to achieve the minimum durability requirements for concrete in buildings. It states “the licensed design professional shall assign exposure classes based on the severity of the anticipated exposure of structural concrete members for each exposure category.”
ACI 318 then goes on to require that concrete mixtures comply with the most restrictive requirements for each exposure class according to a table which includes a maximum w/cm ratio and a minimum specified compressive strength (f’c). ACI 318 also includes minimum requirements of other parameters such as air content, cementitious materials types and limitations regarding the types, the use of calcium chloride admixtures, maximum water-soluble chloride ion content in concrete expressed as percent by weight of the cement and other related provisions.